A multi-stage gearbox having nine forward gears and having one reverse gear is known from DE 10 2008 000 429 A1. The multi-stage gearbox includes four planetary sets, eight rotatable shafts and six shift elements. The first and the second planetary set form a shiftable upstream gear set, whereas the third and the fourth planetary sets constitute a so-called main gear set. The carrier plates or webs of the first and second planetary sets are coupled to one another by one of the rotatable shafts, which is connected to an element of the main gear set. The ring gear of the first planetary set is coupled to the sun gear of the second planetary set by a further shaft of the rotatable shafts, which is detachably connectable by a clutch to a drive shaft.
The sun gear of the first planetary set is couplable via a further shaft of the rotatable shafts to a housing of the multi-stage gearbox by a brake and is connectable by a clutch to the drive shaft. The ring gear of the second planetary set is couplable via a shaft to the housing by a brake. In turn, a further shaft of the rotatable shafts is connected at least to one element of the main gear set and is couplable by the brake to a housing. A further shaft of the rotatable shafts is connected to a further element of the main gear set and is connectable to the drive shaft by a shift element in the form of a clutch, whereas a drive output shaft is connected to at least one further element of the main gear set. At least two of the shift elements of the multi-stage gearbox are in the form of positively engaging shift elements, which are deactivated only during upshifts.
To save fuel, vehicles are equipped with so-called engine start-stop functions, by which, in vehicle ranges close to standstill, and in conjunction with a certain actuation logic, a drive machine can be transferred into a shut-down operating state. Here, by the engine start-stop functions, it is sought for the power flow between the drive machine and the drive output of the vehicle to be produced within short operating times in the region of a gearbox upon starting of the engine.
Furthermore, aside from the engine start-stop functions, vehicles are also configured with so-called sailing functions, by which, during travel, a drive machine in the form of an internal combustion engine is shut down and decoupled from the rest of the drivetrain. When a sailing operating function of said type is active, it is the case inter alia that the existing kinetic energy of a motor vehicle is utilized for forward motion rather than being lost in the form of drag losses. In hybrid vehicles, such sailing operating functions are already widely used, though sailing operating functions are increasingly also being used in conventional motor vehicles with internal combustion engines. By contrast to engine start-stop systems in which the engine is shut down only when a vehicle is at a standstill or close to standstill, it is provided in the case of an active sailing operating function that an engine in the form of internal combustion engine is shut down already during a coasting operating state of a vehicle.
If the drive machine of a vehicle drivetrain configured with the above-described multi-stage gearbox is shut down by an activated engine start-stop function in a vehicle range close to standstill, the two positively engaging shift elements of the multi-stage gearbox are held in the activated operating state, while the frictionally engaging shift elements have to be transferred into the open operating state in order to decouple the drive machine from the drive output to the required extent in the region of the multi-stage gearbox. Upon a restoration of the power flow between the drive machine and the drive output, it is merely necessary for one of the frictionally engaging shift elements to be activated in order to realize a starting transmission ratio in the multi-stage gearbox, in order to be able to produce the operating state required for the resumption of travel of a vehicle configured with the multi-stage gearbox within short operating times.
If the drive output rotational speeds are greater than zero during the operating state of the multi stage gearbox that is produced when the drive machine is shut down and the engine start-stop function is simultaneously active, drag torques which oppose efficiency-optimized utilization of the existing kinetic energy of a vehicle forward motion will rise.